User interfaces for different kinds of electrical apparatuses are nowadays more and more often realized by means of different types of touch sensing devices based on touch sensitive films instead of conventional mechanical buttons. Different kinds of touch pads and touch screens of e.g. mobile phones, portable computers and similar devices are well known examples of these. In addition to the sophisticated and even luxurious user experience achievable, touch sensing devices based on touch sensitive films also provide a superior freedom to the designers continuously trying to find functionally more versatile, smaller, cheaper, lighter, and also visually more attractive devices.
A key element in such touch sensing devices is a touch sensitive film comprising one or more conductive layers configured to serve as one or more sensing electrodes. The general operating principle of this kind of film is that the touch of a user by e.g. a fingertip or some particular pointer device changes the electrical properties of an electrical measuring circuitry to which the touch sensitive film is connected. The actual measuring principle can be e.g. resistive or capacitive, the latter one being nowadays usually considered the most advanced alternative providing the best performance in the most demanding applications.
Capacitive touch sensing is based on the principle that a touch on a touch sensitive film means, from electrical point of view, coupling an external capacitance to the measurement circuitry to which the touch sensitive film is connected. With sufficiently high sensitivity of the touch sensitive film, even no direct contact on the touch sensitive film is necessitated but a capacitive coupling can be achieved by only bringing a suitable pointer to the proximity of the touch sensitive film. The capacitive coupling is detected in the signals of the measurement circuitry.
Conventionally, the capacitive touch sensitive films have been configured as two-layer structures. Typically, each of the two conductive layers is patterned into separate parallel lines or otherwise shaped sensing electrodes. Particularly in the case of line-shaped or elongated sensing electrodes, the electrodes in the two layers are most often arranged orthogonally with respect to each other. A drive signal is supplied into the sensing electrodes of one of the layers while signals capacitively coupled to the other layer are measured via the sensing electrodes of this layer. From operative point of view, the electrodes used for supplying the signal and sensing the capacitive coupling are often called drive electrodes and sense electrodes, respectively. A touch changes the capacitive coupling between the electrodes of the two layers, the change being biggest between the electrodes lying in or near the area of the touch. Usually, the measurement circuitry is arranged to rapidly scan over the sensing electrodes sequentially so that coupling between each supplying/measuring electrode pair is measured.
Recently, also some single-layer capacitive touch sensor configurations have been proposed. In a single-layer configuration, a touch changes electrical coupling of the signals within one single conductive layer and/or between this layer and the ambient. One example of the single-layer approach is disclosed in U.S. Pat. No. 7,477,242 B2. The key feature of the device disclosed therein is use of a conductive polymer as the material of the conductive layer instead of the conductive oxides conventionally used in capacitive touch sensing films of touch screens.
Common for the known touch sensitive films is that the need to properly determine the location of the touch necessitates a high number of separate sensing electrodes in the conductive layers. In other words, the conductive layers are patterned into a network of separate sensing electrodes. The more accurate resolution desired the more complex sensing electrode configuration is needed. One particularly challenging issue is detection of multiple simultaneous touches which, on the other hand, often is one of the most desired properties of the touch sensing devices. Complex sensing electrode configurations and high numbers of single sensing electrode elements complicates the manufacturing process as well as the measurement electronics of the touch sensing device.
In touch screens, in addition to the touch sensing capability, the touch sensitive film must be optically transparent to enable use of the film on top of a display of an electronic device, i.e. to enable the display of the device to be seen through the touch sensitive film. Moreover, transparency is also very important from the touch sensitive film visibility point of view. Visibility of the touch sensitive film to the user of e.g. an LCD (Liquid Crystal Display), an OLED (Organic Light Emitting Diode) display, or an e-paper (electronic paper) display seriously deteriorates the user experience. So far, transparent conductive oxides like ITO (Indium Tin Oxide) have formed the most common group of the conductive layer materials in touch sensitive films. However, from the visibility point of view, they are far from an ideal solution. The high refractive index of e.g. ITO makes the patterned sensing electrodes visible. The problem is emphasized as the sensing electrode patterning becomes more complicated.
One promising new approach in touch sensitive films is found in layers formed of or comprising networked nanostructures. In addition to a suitable conductivity performance, a layer consisting of networks of e.g. carbon nanotubes (CNT), or carbon NANOBUDs having fullerene or fullerene-like molecules covalently bonded to the side of a tubular carbon molecule (NANOBUD® is a registered trade mark of Canatu Oy), are clearly less visible to a human eye than e.g. transparent conductive oxides like ITO. Besides, as is well known, nanostructure-based layers can possess flexibility, mechanical strength and stability superior in comparison with e.g. ITO.
One nanostructure-based solution is reported in US 2009/0085894 A1. According to the description thereof, the nanostructures can be e.g. different types of carbon nanotubes, graphene flakes, or nanowires. Doping of the film is mentioned as a means for increasing the electrical conductivity thereof. Both two-layer configurations based on mutual capacitance and single-layer self-capacitance approaches are discussed. Multiple touch detection is stated to be possible by means of the films disclosed. However, also this document involves the common problem of very complex electrode and measurement circuitry configurations.
Other touch screen solutions based on nanostructure networks are disclosed in US 2008/0048996 A1. The document mainly discusses layers of nanostructure networks in touch sensitive devices relying on resistive measuring principles. Also a capacitive, apparently single layer approach with a non-patterned conductive layer is shortly discussed and illustrated in a figure, however more or less just as a principle of a desired target without any real description about the implementation thereof in practice.
To summarize, there is still a strong demand in the market for further enhanced touch sensitive films and touch sensing devices, preferably enabling single-layer capacitive operation principle with a simple sensing electrode configuration and multi-touch sensing capability.
Moreover, there is also a need in the market to provide touch sensitive films and touch sensing devices with versatile properties enabling detection of various types of pointers or other objects coupling to the touch sensitive film in various ways. For example, it would be advantageous if the touch sensitive films and touch sensing devices could be used to detect not only objects being coupled to the touch sensitive film capacitively but also, for example, objects coupled to the touch sensitive film inductively.